The Scientist should create a second group of participants with ear infections who do not use any ear drops; option D.
An incubation period is a period during which someone has an infection, but is not showing symptoms; option B.
A control is a group in an experiment or research which do not receive the same treatment as the test group during the research.
In order for the scientist to improve the ability to test if the new medication effectively treats ear infections, he should set up a control group of participants with ear infections who do not use any ear drops.
In diseases, an incubation period is a period during which someone has an infection, but is not showing symptoms.
Incubation periods are usually when the disease causing organisms multiply.
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